1. Field of the Invention
The present invention relates to a method for producing printed circuitry having built-up areas coexisting with narrow conductors or leads and/or narrow spacing between the conductors or leads.
2. Description of the Prior Art
The requirements for increasing the density of electric circuitry or to achieve a desired electrical characteristic, e.g., impedance, generally require that the conductors and the spacing between conductors become as narrow or small as possible. At the same time, it is also necessary to provide sufficient build-up of material for use in forming plated-through holes in two sided and multi-layer printed wiring boards and in developing thicker conductors, such as used as power busses. These two needs, that is, narrow conductors and thicker built-up areas, are difficult to obtain simultaneously.
In a first conventional process involving panel plating, copper is added to the entire outer surface of the printed wiring board; however, the thick plating obtained from panel plating makes it difficult to etch narrow conductors. The difficulty arises because, as the thickness of copper increases, the amount of undercutting resulting from the etching process increases. Therefore, for narrow conductors, the etching may cause sufficient undercutting as to leave too little attachment of the conductor to the substrate or to significantly remove sufficient lead material as to adversely affect the cross-section and electrical characteristics of the conductors.
In the method of pattern plating, copper is added to the conductor pattern which makes it difficult to maintain narrow spacing. For example, if a thin resist it utilized, the problem of plating overhang results, that is, plating builds up and over the resist at the edges of the resist, which also decreases the spacing between the conductors. The major problem is that the geometry cannot be controlled which, in general, leads to a lack of control of the electrical characteristics. Such uncontrolled geometry decreases spacing between conductors, increases the possibility of shorts between lines, decreases the amounts of voltage which can be carried because of the decreased insulation, makes it difficult to control the impedances between the lines, and increases capacitance between lines. A further problem results from an increase in failure due to the existence of organic contaminants which may be trapped under or between the lines, resulting, for example, in electrical leakage paths.